A harmless prank that can be put on a Mac. Make sure to run it in screen.
This was written on an iPhone.
Run the following commands:
screen -S virus curl https://raw.github.com/simplyianm/osx-prank/master/prank.sh | sh
Then close the terminal window.
NOTE: This prank is difficult to execute quickly since this URL is so long. Maybe if I could host it from a domain in the future...